1. Field of Invention
The present invention relates to a semiconductor fabricating method. More particularly, the present invention relates to a method of forming interconnects.
2. Description of Related Art
Tungsten plugs are widely used for interconnections of Very Large Scale Integration (VLSI). Because of the poor step coverage ability of tungsten, holes are often formed in the tungsten plug while forming a tungsten plug. Therefore, in the step of performing chemical mechanical polishing (CMP) method, slurry often fills the holes in the plug. The slurry easily reacts with tungsten, and thus the reliability of tungsten plug is reduced.
FIGS. 1A through 1C are cross-sectional views of a portion of a semiconductor device showing steps in a conventional process for forming a plug.
In FIG. 1A, a substrate 100 is provided. A metallic layer 104 is formed on the substrate 100. The metallic layer 104 is used to electrically couple with the other regions. A dielectric layer 102 is formed on the substrate 100. The dielectric layer 102 is etched by conventional microphotography and etching processes. An opening 106 is formed in the dielectric layer 102. The opening 106 exposes the metallic layer 104.
In FIG. 1B, a tungsten layer 122 is formed on the substrate 100. The surface of the tungsten layer 122 is about 5000 .ANG. higher than the surface of the dielectric layer 102. In other words, the thickness 124 of the tungsten layer 122 is about 5000 .ANG.. The tungsten layer 122 is sufficient to fill the opening 106. Because of the poor step coverage ability of the tungsten, a hole 108 is formed in the tungsten plug.
In FIG. 1C, a portion of the tungsten layer 122 on the surface of the dielectric layer 102 is removed by using chemical mechanical polishing. A tungsten plug 122a is formed. An over-polishing method is performed to prevent any portion of the tungsten layer 122 from remaining on the dielectric layer 102. A dished surface 152 is formed on the surface of the tungsten plug 122a. If the hole 108 is large enough, it is easy for the hole 108 to connect with the dished surface 152 to form a hole opening 130. The hole opening 130 exposes the hole 108. At the same time, slurry 132 easily fills the hole 108 through the hole opening 130. The slurry 132, which fills the hole 108, may react with the tungsten plug 122a. Hence, the hole 108 is enlarged. The quality and reliability of the tungsten plug 122a thus are reduced. Furthermore, the cost of the chemical mechanical polishing method is expensive. Using chemical mechanical polishing to remove a tungsten layer of about 5000 .ANG. in thickness costs a lot.